Optical micromanipulation using optical trapping is a powerful and versatile tool for studies in colloidal and biological science. An optical trap can be formed using two counter propagating diverging beams due to a combination of optical refraction and optical scattering, as described, for example, in the article “Demonstration of a Fibre-Optical Light-Force Trap” by Constable et al., Opt. Lett. 18, 1867 (1993). The trap described by Constable et al uses two optical fibers fibres that deliver light to a trap region in a counter-propagating geometry. This dual beam trap may be easily integrated into micro-fluidic devices, has a large capture range, does not use tightly focused light, and allows trapping and imaging to be decoupled.
In the last decade, photonic crystal fibers (PCF) have become available. Photonic-crystal fibers are based on the properties of photonic crystals. These are able to confine light in hollow cores or with confinement characteristics not possible in conventional optical fiber. Categories of PCF include photonic bandgap fibers that confine light by band gap effects, holey fibers, which use air holes in their cross-sections, hole-assisted fibers, which guide light by a conventional higher-index core modified by the presence of air holes, and Bragg fibers that are formed by concentric rings of multilayer film. PCFs are normally uniform along their length, but include from two or more materials, most commonly arranged periodically over much of the fiber cross-section, as shown in FIG. 1.
PCFs can be engineered to have vastly different properties compared to conventional silica fibers, see for example P. Russell, Science 299, 358 (2003). With the appropriate design of the crystal lattice, fibers can be designed so that large core sizes (much larger than standard single mode fibers) may confine any wavelength of light in a single mode. These fibers are known as endlessly single mode photonic crystal fibers (ESM-PCF) or large mode area photonic crystal fibers (LMA-PCF).